The identification of small organic molecules that affect specific biological functions is an endeavor that impacts both biology and medicine. Such molecules are useful as therapeutic agents and as probes of biological function. In but one example from the emerging field of chemical genetics, in which small molecules can be used to alter the function of biological molecules to which they bind, these molecules have been useful at elucidating signal transduction pathways by acting as chemical protein knockouts, thereby causing a loss of protein function. (Schreiber et al., J. Am. Chem. Soc., 1990, 112, 5583; Mitchison, Chem. and Biol., 1994, 1, 3) Additionally, due to the interaction of these small molecules with particular biological targets and their ability to affect specific biological function, they may also serve as candidates for the development of therapeutics. One important class of small molecules, natural products, which are small molecules obtained from nature, clearly have played an important role in the development of biology and medicine, serving as pharmaceutical leads, drugs (Newman et al., Nat. Prod. Rep. 2000, 17, 215–234), and powerful reagents for studying cell biology (Schreiber, S. L. Chem. and Eng. News 1992 Oct. 26, 22–32).
Because it is difficult to predict which small molecules will interact with a biological target, and it is oftent difficult to obtain and synthesize efficiently small molecules found in nature, intense efforts have been directed towards the generation of large numbers, or libraries, of small organic compounds, often “natural product-like” libraries. These libraries can then be linked to sensitive screens for a particular biological target of interest to identify the active molecules.
One biological target of recent interest is histone deacetylase (see, for example, a discussion of the use of inhibitors of histone deacetylases for the treatment of cancer: Marks et al. Nature Reviews Cancer 2001, 1, 194; Johnstone et al. Nature Reviews Drug Discovery 2002, 1, 287). Post-translational modification of proteins through acetylation and deacetylation of lysine residues has a critical role in regulating their cellular functions. HDACs are zinc hydrolases that modulate gene expression through deacetylation of the N-acetyl-lysine residues of histone proteins and other transcriptional regulators (Hassig et al. Curr. Opin. Chem. Biol. 1997, 1, 300–308). HDACs participate in cellular pathways that control cell shape and differentiation, and an HDAC inhibitor has been shown effective in treating an otherwise recalcitrant cancer (Warrell et al. J. Natl. Cancer Inst. 1998, 90, 1621–1625). Nine human HDACs have been characterized ((a) Taunton et al. Science 1996, 272, 408–411; (b) Yang et al. J. Biol. Chem. 1997, 272, 28001–28007. (c) Grozinger et al. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 4868–4873. (d) Kao et al. Genes Dev. 2000, 14, 55–66. (e) Hu et al. J. Biol. Chem. 2000, 275, 15254–15264. (f) Zhou et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10572–10577) and two inferred (Venter et al. Science 2001, 291, 1304–1351) these members fall into two related classes (class I and II). To date, no small molecules are known that selectively target either the two classes or individual members of this family ((for example ortholog-selective HDAC inhibitors have been reported: (a) Meinke et al. J. Med. Chem. 2000, 14, 4919–4922; (b) Meinke, et al. Curr. Med. Chem. 2001, 8, 211–235).
Clearly, it would be desirable to develop compounds capable of selectively targeting either of the two classes or individual members of this family. Additionally, it would be desirable to identify novel compounds capable of acting as probes of biological function.